Conventionally, the following coil devices, which utilize a magnetic core having gaps used for the transformer or the choke coil, have been employed such as: 1) the coil device utilizing an opposed pair of E-shaped cores (disclosed in Japanese Patent Publication No. 50372/1980) made from magnetic materials such as ferrite which is made from the baked oxide such as nickel, iron or zinc, wherein gaps are provided on ends of the center of a leg in order to prevent magnetic saturation, 2) a combination of E-shaped and I-shaped cores (disclosed in Japanese Patent Publication No. 24363/1981), wherein the gaps are provided in the end of the center leg of the E-shaped core, and 3) the coil device in which the coil is wound on the drum-shaped core.
However, with the conventional invention in which a wire is wound on the magnetic core having the gaps, it often causes errors in inductance (that is different from the expected value), which is most often derived from errors in the dimension of the magnetic core, errors during the production of the gaps and errors in magnetic permeability of the core. For example, when an effective permeability is 100, the errors (variation) of the inductance of the choke coil is .+-.21% in E-E type, and is .+-.16% in E-I type. On the other hand, with the invention using a drum core only, the error is relatively small (.+-.6%). However, it has a problem that leakage flux becomes extremely large.